Conventionally, for example, Patent Document 1 discloses an air-fuel ratio learning control device for an internal combustion engine that includes a fuel injection valve for each of two intake ports provided in each cylinder. In order to separately learn and correct deviations of injection characteristics of the two fuel injection valves in the each cylinder, this conventional control device executes following control. More specifically, the two fuel injection valves in the each cylinder are arranged to be temporarily driven one after another, and, based on air-fuel ratio feedback correction amounts before and after the other injection valve stops injection, the deviation of the injection characteristic of the each fuel injection valve in the cylinder to be learned is learned independently by using output of an air-fuel ratio sensor disposed in an exhaust passage.
As in the internal combustion engine described in Patent Document 1, in the internal combustion engine that includes the fuel injection valve for each of the plural intake ports provided independent of each other in each cylinder, an amount of fuel that flows together with air from the each intake port into the cylinder can vary in each cycle due to factors such as adhesion of the fuel to the each intake port or the like and the deviation in the injection characteristic of the each fuel injection valve. However, in a method described in Patent Document 1, if the plural (two, for example) fuel injection valves in the each cylinder are simultaneously driven, it is impossible to detect the variation in the amount of the fuel for the each intake port per cycle that is injected by the each fuel injection valve and flows from the each intake port into the cylinder. In addition, as in the method described in Patent Document 1, if the fuel is injected only from one of the fuel injection valves to learn the deviation in the injection characteristic thereof, it is concerned that the fuel supplied to the cylinder becomes unbalanced (becomes inconsistent) and that fuel economy and exhaust emission (THC, CO, and the like) may be worsened.
In addition to the above document, the applicant recognizes the following documents to be related to the present invention.